Method and system for providing differentiated quality of service diversity ALOHA

ABSTRACT

An approach for providing access in a communications system is disclosed. A network node (or terminal) generates duplicate bursts. The network node then transmits the duplicate bursts over a contention channel according to a pre-determined quality-of-service (QoS) designation. The contention channel can be established based on a diversity ALOHA protocol. The present invention has particular applicability to a radio communications system, such as a satellite network that provides data communication services.

FIELD OF THE INVENTION

The present invention relates to a communications system, and is moreparticularly related to multiple access communications systems.

BACKGROUND OF THE INVENTION

Wireless communications systems (e.g., satellite networks) provide apervasive and reliable infrastructure to distribute voice, data, andvideo signals for global exchange and broadcast of information. Thesecommunications systems have emerged as a viable option to terrestrialcommunications systems. Unlike terrestrial networks, such systems aremore susceptible to service disruptions stemming from changing channelconditions, such as fading because of weather disturbances.Additionally, such systems cannot readily increase capacity, and thus,rely on various techniques to enhance spectral efficiency. Further,because of the diversity of communications needs of subscribers,subscribers demand that these wireless communications systems provideassurances with respect to availability, reliability, and throughput ina cost-effective manner. If a service provider supplies a uniform levelof service, then more demanding subscribers would not be satisfied for a“standard” service guarantee, even though such subscribers are willingto pay a greater premium for better service; under this scenario, theservice provider is foregoing revenue. However, if the uniform level ofservice is greater than what the less demanding subscribers seek, thenthese subscribers are essentially “overpaying” for the level of service,as they may be willing to pay less for fewer guarantees regardingnetwork performance.

Wireless communications systems often employ multiple terminals to sharea resource of spectrum; such a system is referred to as a multipleaccess system. Many methods of providing multiple access are deployed inexisting communications systems; such as a Frequency Division MultipleAccess (FDMA) system, and a Time Division Multiple Access (TDMA). WithFDMA, a band of spectrum is supplied to each terminal based on frequencyassignments, thereby allowing the terminals to share the spectrum. TheTDMA scheme permits sharing the spectrum based on time; that is, when aterminal allowed to transmit.

Many different methods have been employed to allow terminals to performmultiple access in TDMA and FDMA/TDMA systems. These multiple accesspolicies are typically evaluated in terms of their efficiency and theirlatency characteristics. Often, as the efficiency increases, the latencycharacteristics degrade. Efficiency, in this context, means the totalnumber of usable bits carried divided by the total of each channel'sinformation rate. An important example of a latency characteristic isthe percentage of transmissions which are successfully delivered in lessthan a specific threshold latency, for example, 1200 msec

One multiple access scheme is referred to as Slotted ALOHA, which isconventionally used in a satellite system. With slotted ALOHA, thechannels in a FDMA/TDMA system or the single channel in a TDMA system isdivided into fixed size timeslots, referred to as ALOHA slots. When aterminal in the satellite system has data to transmit, the terminalselects an ALOHA slot at random and transmits the data within the ALOHAslot. The transmission within a slot is referred to as a burst.Frequently, when the system is lightly loaded, the terminal selects aslot that is not used by another terminal in which case the burst isconsidered successful. The term “collision” refers to the event in whichtwo or more terminals attempt to transmit in the same selected slot.These transmissions interfere with each other, and in most cases none ofthe bursts transmitted in the slot are successfully delivered. When aterminal detects that its burst has failed, which typically occursbecause of a collision, the terminal randomly picks another slot andretransmits. Provided the system is not overloaded, the terminal shouldwithin a small number of tries have its transmission succeed.

Based on the foregoing, there is a need for a radio communicationssystem that provides a quality-of-service (QoS) mechanism in a sharedmedium using a multiple access scheme. There is also a need to enhancespectral efficiency. There is a further need to provide cost-effectivenetwork services to subscribers that are tailored to the subscribers'needs. Therefore, an approach for supporting differentiated networkservices is highly desirable.

SUMMARY OF THE INVENTION

These and other needs are addressed by the present invention, wherein anapproach is provided for performing a contention scheme that supportsdifferentiated quality of service levels. Under one embodiment of thepresent invention, terminals communicate according to a diversity ALOHAprotocol, in which quality of service levels correspond to the amount ofdiversity. In an exemplary embodiment, a communications system withtwo-levels of service (e.g., basic and premium) can specify that thebasic terminals operate with a diversity of two slots (i.e.,transmitting each portion of data in two different, duplicate bursts),while the premium service can specify a diversity of four slots (i.e.,transmitting each portion of data in three different, duplicate bursts).Additionally, in an exemplary embodiment, the contention channel isestablished according to a Frequency Division Multiple Access(FDMA)/Time Division Multiple Access (TDMA) scheme for providingcommunication in a satellite communications system. The abovearrangement advantageously provides a mechanism to supportQuality-of-Service (QoS) services over a contention channel.

According to one aspect of the present invention, a method for providingaccess in a communications system is disclosed. The method includesgenerating a plurality of bursts, wherein one of the bursts is aduplicate of another one of the bursts. The method also includestransmitting the duplicate bursts over a contention channel according toa pre-determined quality-of-service (QoS) designation.

According to another aspect of the present invention, a network devicefor providing access in a communications system is disclosed. Thenetwork device includes logic configured to determine a service levelassociated with transmission of a burst. The network device alsoincludes a transceiver configured to transmit multiple copies of theburst over a contention channel according to the service level.

According to another aspect of the present invention, a system forproviding access in a communications system is disclosed. The systemincludes means for generating a plurality of bursts, wherein one of thebursts is a duplicate of another one of the bursts. The system alsoincludes means for transmitting the duplicate bursts over a contentionchannel according to a pre-determined quality-of-service (QoS)designation.

According to another aspect of the present invention, acomputer-readable medium carrying one or more sequences of one or moreinstructions for providing access in a communications system isdisclosed. When executed by one or more processors, the instructionscause the one or more processors to perform the step of generating aplurality of bursts, wherein one of the bursts is a duplicate of anotherone of the bursts. Another step includes transmitting the duplicatebursts over a contention channel according to a pre-determinedquality-of-service (QoS) designation.

According to another aspect of the present invention, a method forproviding access to a contention channel for a plurality of terminals ina communications system is disclosed. The method includes specifying aplurality of service levels for the plurality of terminals. The methodalso includes executing a communication protocol to provide thecontention channel, wherein one of the plurality of terminalsconcurrently transmits a plurality of bursts according to thecorresponding service level, and one of the bursts is a duplicate ofanother one of the bursts.

According to another aspect of the present invention, acomputer-readable medium carrying one or more sequences of one or moreinstructions for providing access to a contention channel for aplurality of terminals in a communications system is disclosed. Whenexecuted by one or more processors, the instructions cause the one ormore processors to perform the steps of specifying a plurality ofservice levels for the plurality of terminals, and executing acommunication protocol to provide the contention channel. One of theplurality of terminals transmits a plurality of bursts according to thecorresponding service level, and one of the bursts is a duplicate ofanother one of the bursts.

According to another aspect of the present invention, a method forcommunicating over a shared medium is disclosed. The method includesreceiving one or more bursts over a contention channel of the sharedmedium from a terminal, wherein the bursts are duplicates, the quantityof bursts being dependent on a pre-determined quality-of-service (QoS)designation associated with the terminal. The method also includesselectively discarding all but one of the bursts.

According to another aspect of the present invention, acomputer-readable medium carrying one or more sequences of one or moreinstructions for communicating over a shared medium is disclosed. Whenexecuted by one or more processors, the instructions cause the one ormore processors to perform the step of receiving one or more bursts overa contention channel of the shared medium from a terminal, wherein thebursts are duplicates. The quantity of bursts is dependent on apre-determined quality-of-service (QoS) designation associated with theterminal. Another step includes selectively discarding all but one ofthe bursts.

Still other aspects, features, and advantages of the present inventionare readily apparent from the following detailed description, simply byillustrating a number of particular embodiments and implementations,including the best mode contemplated for carrying out the presentinvention. The present invention is also capable of other and differentembodiments, and its several details can be modified in various obviousrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawing and description are to be regardedas illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a diagram of a communications system capable of utilizing amultiple access scheme, according to an embodiment of the presentinvention;

FIG. 2 is a diagram of a satellite communications system capable ofproviding a Quality-of-Service (QoS) mechanism over a contentionchannel, according to an embodiment of the present invention;

FIGS. 3A-3C are flowcharts of a process for supporting QoS services inthe system of FIG. 2;

FIG. 4 is a graph showing the performance of a slotted ALOHA systemversus a diversity ALOHA system;

FIG. 5 is a graph showing the performance of the system of FIG. 2,wherein a Diversity ALOHA scheme provides 4 slots and 2 slots forpremium service and basic service, respectively;

FIG. 6 is a graph showing the performance of the system of FIG. 2,wherein a Diversity ALOHA scheme provides 3 slots and 2 slots forpremium service and basic service, respectively;

FIG. 7 is a graph showing the performance of the system of FIG. 2,wherein a Diversity ALOHA scheme provides 2 slots and 1 slot for premiumservice and basic service, respectively; and

FIG. 8 is a diagram of a computer system that can support QoS services,in accordance with an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A system, method, device, and software for supporting multiple qualityof service levels over a contention channel are described. In thefollowing description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It is apparent, however, to oneskilled in the art that the present invention may be practiced withoutthese specific details or with an equivalent arrangement. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

Although embodiments of the present invention are explained with respectto a satellite communications system, it is recognized that the presentinvention can be practiced in any type of communications system thatutilizes multiple access protocols, such as slotted ALOHA. For example,slotted ALOHA can be used in terrestrial communications systems; forexample, TDMA digital cellular systems to carry signaling traffic fromthe cell phones to the cellular base stations.

FIG. 1 is a diagram of a communications system capable of utilizing amultiple access scheme, according to an embodiment of the presentinvention. A communications system 100 includes one or more networknodes 101, 103 that provide access for connected hosts 105, 107,respectively, to a shared medium 109. The share medium 109 can be anytype of data network that supports multiple access communicationsprotocols (e.g., slotted ALOHA, diversity ALOHA, etc.). Also, attachedto the shared medium 109 is a hub node 111 that can be responsible forestablishing and managing the contention channels with the shared medium109.

As shown, the node 101 has a Quality-of-Service (QoS) logic 101 a fordetermining the designated quality of service level (e.g., premiumservice or basic service) for the particular node 101. An access logic101 b determines and sets the necessary parameters associated with amultiple access protocol to implement and maintain the determinedquality of service level. For example, in the case of a diversity ALOHAprotocol, the access logic 101 b can specify use of a larger amount oftime-slots (e.g., 4 or 3) for the premium service compared with that ofthe basic service (e.g., 2 slots, or even 1 slot). A transceiver 101 ccan transmit the bursts over the shared medium. It is noted that thenode 103 can also be equipped with QoS logic and access logic, similarto that of the network node 101. Although the logic 101 a, 101 b areshown as separate components, it is recognized that the functionalitiesof these logic 101 a, 101 b can be combined as a single component.Further, the logic 101 a, 101 b can be implemented in hardware and/orsoftware.

The system 100, according to one embodiment of the present invention,can be deployed as a satellite communications system, as described withrespect to FIG. 2.

FIG. 2 is a diagram of a satellite communications system capable ofproviding a Quality-of-Service (QoS) mechanism over a contentionchannel, according to an embodiment of the present invention. Under thisscenario, a radio communication system 200 supports multiple networknodes (i.e., terminals) 201, 203, 205, 207, 209, which can communicatewith each of the terminals as well as a hub station 213 via ageosynchronous satellite 211. The terminals 201, 203, 205, 207, 209 areVery Small Aperture Terminals (VSATs) capable of supplying datacommunication services for connected hosts (not shown). Slotted ALOHAhas been employed in many different communications system includingsatellite communications systems. It is often used in Very SmallAperture Terminal (VSAT) satellite communications systems. A VSAT systemtypically has approximately an 800 ms round-trip delay. Slotted ALOHAprovides, when properly tuned, a relatively short response time in thata transmission may begin immediately without waiting for a request forbandwidth to be sent to the hub station. Slotted ALOHA is used both toactually carry traffic from the VSAT to the hub and also to carry aninitial bandwidth request of more complicated multiple access policiessuch as reservation policies.

In an exemplary embodiment, the system 200 utilizes an access schemethat combines both FDMA and TDMA. In such an FDMA/TDMA system 200, thespectrum is first divided by frequency into several channels 215, 217;the system 200 then employs TDMA to share the individual channels 215,217 in form of time slots among the terminals 201, 203, 205, 207, 209.By way of example, two contention channels 215, 217 are shown, providingFDMA/TDMA slotted ALOHA multiple access.

The satellite 211 relays the spectrum from multiple FDMA channels 215,217 received from anywhere under the “foot print” of the satellite 211on a different frequency back to back to earth anywhere under thesatellite's foot print. The hub station 213 is thus configured toreceive the FDMA channels 215, 217. According to an embodiment of thepresent invention, the FDMA channels 215, 217 are organized in time intofixed length ALOHA slots. When a terminal 201, 203, 205, 207, 209 hasdata to transmit, the terminal 201, 203, 205, 207, 209 selects a slot atrandom and transmits a burst containing the data into the selected slot.When multiple terminals 201, 203, 205, 207, 209 select an identical timeslot, as is the case with the terminals 207, 209 into the channel 217 onslot 5, a collision occurs, thereby requiring the terminals 207, 209 toretry (i.e., attempt another transmission).

Diversity ALOHA provides an enhancement to slotted ALOHA. With diversityALOHA, when a terminal needs to transmit, the terminal select not asingle slot, but multiple slots at random in which to transmit. Thetransmission is considered successful when any of the transmissionssucceed. The use of Diversity ALOHA increases, compared to slottedALOHA, the maximum efficiency achievable for a given latency threshold.

A number schemes within the general family of slotted ALOHA multipleaccess exists whereby a terminal may determine whether one of its burstswas successful. In some cases, the system provides an indication to theterminals of the status of each of the ALOHA slots. That statusindicates whether the slot contained a successful burst. A terminal candetermine whether one of its bursts has been transmitted successful bychecking the status of that burst's slot. In other cases an automaticrepeat request (ARQ) protocol is used and a terminal retransmits a burstif a timeout expires without an acknowledgement for the burst beingreceived or retransmits after a negative acknowledgement for the burstis received.

With diversity ALOHA, however, it is recognized that there is a higherprobability that the same data will be delivered more than once.Additionally, a high probability exists that data will be delivered outof sequence; in other words, the order of the data that is receiveddiffers from the order that the data was originally intended to bedelivered. As a result, the data sent in a burst provides informationthat identifies the burst's data in such a way that the receiver ofthose bursts may discard duplicate transmissions. This information, inan exemplary embodiment, can be the source address of the transmitterand a sequence number. The use of a source address and sequence numberallows a receiver to not only eliminate duplicate transmissions, butalso return out-of-sequence data back to their original intended order.

Because the system 200 can support QoS services, a service provider ofthe system 200 can increase total revenue by charging more for a“premium” quality of service. By contrast, the conventional use ofslotted ALOHA and diversity ALOHA has the drawback that a uniformquality of service in terms of latency experienced is given to theterminals in the system. With conventional slotted ALOHA and diversityALOHA system, support for QoS services can be implemented by dedicatinga subset of the slots to the terminals associated with premium users insuch a way that those slots were managed to have lower utilization (andthus offer better latency characteristics). Unfortunately, themanagement of these subsets of slots is very difficult, and therefore,error prone, especially when the number and mix of users vary. Thisapproach suffers from another drawback in that it is inefficient,particularly when the fraction of “premium” terminal is small.

FIGS. 3A-3C are flowcharts of a process for supporting QoS services inthe system of FIG. 2. This process represents one embodiment of thepresent invention; it is noted that various embodiments exists such thatdifferent quality of service levels (i.e., two or more levels) aresupported over other types of networks. For the purposes of explanation,the process for supplying different quality service levels is describedfor two service levels (e.g., premium and basic) within the system 100employing diversity ALOHA. That is, the diversity of the terminal'stransmission is a function of its service quality level. It isunderstood, according to various embodiments of the present invention,that any number of service levels can be implemented, such that thediversity increases with higher quality of service. In step 301, thenode 101 receives data from one of the hosts 105; the data is preparedfor transmission within one or more bursts over a contention channelwithin the shared medium 109. The QoS logic 101 a determines, as in step303, whether this packet for this node 101 is associated with a premiumservice; if not, the diversity parameter is set to effect a basicquality of service (e.g., 1 or 2 slots), per step 305. Differentcategories of traffic carried by a single node may be carried bydifferent quality of service levels. If the service level of the node101 is premium, then, per step 307, the diversity parameters areappropriately set (e.g., 2-4 slots).

Prior to transmission, the node 101 initializes an Attempt counter and aBurst History counter, per step 309. The Attempt counter tracks thenumber of transmission attempts (i.e., “TRIES”). The Burst Historycounter reflects the number of duplicate burst transmissions by the node101 for the subject burst, such that the node 101 continues to transmitduplicate bursts until the proper diversity parameter is satisfied.

Next, as shown in FIG. 3B, in step 311, the number of attempts (TRIES),as captured by the Attempt counter, is compared with a predeterminedmaximum value (i.e., “MAX TRIES”). If the value of TRIES meets orexceeds the threshold, MAX TRIES, then an unsuccessful transmission ofthe data is declared (per step 313). If, however, the number of attemptsis less than the threshold, MAX TRIES, then the QoS logic 101 a checks,as in step 315, whether the Burst History counter value is greater thanor equal to the diversity parameter (as set per step 305 or step 307).If the Burst History counter value is less than the diversity parameter,then the node 101 selects an ALOHA slot from the next select rangeslots, per step 317. In step 319, the node 101 transmits the burst intothe selected slot, and the Burst History counter is incremented (perstep 321). Thereafter, step 315 is repeated until the value of the BurstHistory counter is greater than the diversity parameter, therebyensuring that the designated quality of service level corresponds to thediversity.

In step 323, the source node 101 determines whether anyone of theduplicate bursts is received by the destination node (e.g., node 103),as shown in FIG. 3C. If at least one of the duplicate bursts is received(as determined by step 325), then the data transmission is deemedsuccessful (per step 327). However, if no burst is received, then acollision is assumed, and the Attempt counter is incremented, as in step329; thereafter, step 311 is repeated.

According to one embodiment of the present invention, each differentblock of data to be transmitted in a burst is sent along with datauniquely identifying the data. This identifier information can be sentunchanged along with the data in each burst of each try (or retry) ofthe data. In an exemplary embodiment, the identifier information caninclude the identification (ID) of the node (or terminal). Consequently,this allows the receiving node 103 (i.e., receiver) to detect anddiscard duplicate transmissions of the data.

Further, each different block of data can also transmitted, in anexemplary embodiment, along with data uniquely identifying the orderthat the data should be delivered (i.e., sequence information). Thesequence information specifies the ordering of the data, therebypermitting the receiver to organize the received data back in itsoriginal order. This sequence information, in an exemplary embodiment,can include a modulo N sequence number, where N is of sufficient size toprovide unique sequence numbers for a given series of bursttransmissions. In other words, N is set such that the receiving nodeneed ever handle different data with the same sequence number.

The above two mechanisms thus permit the receiver (e.g., the destinationnode 103) to reassemble a terminal's data and deliver it in orderwithout duplicates. Further, for the source node 101 can determinewhether its bursts were successfully received by the information that issent back from the receiver. For example, the receiver can send back theidentifier information and sequence information to indicate that thebursts have been successfully received. Alternatively, the receiver cansimply acknowledge the transmission by sending back the highest sequencenumber from each terminal which had a burst successfully received.Further, the receiver can send back, for each ALOHA slot, an indicationof whether a burst was successfully received in the slot.

The effectiveness of the approach of FIG. 3 is shown in the graphs ofFIGS. 5-7, which illustrate the performance characteristics of the QoSmechanisms of various embodiments of the present invention. As abaseline, the performance of a conventional slotted ALOHA system areshown in FIG. 4.

FIG. 4 is a graph showing the performance of a slotted ALOHA systemversus a diversity ALOHA system. Slotted ALOHA is frequently used insatellite to carry the initial transmission of a terminal along withrequests for reserved bandwidth to carry subsequent data. A trafficmodel captures this process by simulating a terminal that waits a randomperiod of time, and then generates a single request. When the singlerequest's transmission completes, the process repeats with the terminalwaiting another random period of time. A simulated terminal generates arequest that is to be transmitted according to a uniform random intervalbetween 0 and 2000 milliseconds (msec) after start of the simulation orcompletion of the attempt to transmit the previous burst. In thissimulation, a burst is assumed to be successful, unless it collides withone or more other bursts—in which case all bursts involved in thecollision fail. In satellite systems, one way of evaluating a multipleaccess scheme is to determine the maximum efficiency the scheme candeliver and still have, for example, 95% of all transmission take placein less than 1200 msec.

Furthermore, for simplicity, a single channel is used with one ALOHAslot per msec. The round-trip time refers to the time a terminal takesto determine whether its previous transmission was successful. For thissimulation, the round-trip time is assumed to be 800 ms, which models atypical VSAT satellite communications system. Additionally, the load onthe channel is increased by running the simulation multiple times withan increasing number of users for each run.

The simulation shows the percentage of transmissions for which theround-trip time is less than 1200 msec as a function of ALOHA channelload for a slotted ALOHA function 401 and for a diversity 2 ALOHAfunction 403, that is, where a terminal sends each burst twice. Both theslotted ALOHA function 401 and the diversity ALOHA function 403 have thecharacteristic that when overloaded the actual amount of traffic carriedfalls off; this is seen as the functions 401, 403 curve, “circlingback”. FIG. 4 illustrates that diversity ALOHA 403 provides moretransactions under the 1200 msec response time threshold when thechannel is not overloaded. Accordingly, the diversity ALOHA schemeprovides a higher capacity for a given percentage of transmissions thatmust meet a round-trip delay (e.g., 1200 msec).

FIG. 5 is a graph showing the performance of the system of FIG. 2,wherein a Diversity ALOHA scheme provides 4 slots and 2 slots forpremium service and basic service, respectively. By way of example, twogrades of service quality levels are provided in this simulation: basicand premium. It is assumed that 90% of the terminals serve basic users,and 10% are premium terminals. MAXTRIES is set to 10 and SELECT RANGE isset to 100 slots.

For an individual request, the key performance measurements, in anexemplary embodiment, include whether the request was successfullycarried within the MAX TRIES, round-trip delay, and whether theRound-Trip Delay was under a specified threshold (e.g., 1200 msthreshold). The Round-Trip Delay is measured in terms of the number ofmilliseconds from when the request was generated until it is known bythe sender to be successfully transmitted. This corresponds to theround-trip delay seen by an actual request/response transaction whenthere is zero server delay.

These individual request performance measurements are totaled to producethe following metrics for each simulation run: Channel Load, BasicAverage Delay, Premium Average Delay, Basic Under-Threshold Percentage,and Premium Under-Threshold Percentage. The Channel Load can be computedfrom the total number of Success Requests divided by the total number ofALOHA slots. The Basic Average Delay is the average delay from all ofthe Success requests from basic terminals. The Premium Average Delay isthe average delay from all of the Success requests from premiumTerminals. The Basic Under-Threshold Percentage is the percentage ofbasic terminal requests, which were received under the 1200 msthreshold. The Premium Under-Threshold Percentage is the percentage ofpremium terminal requests, which were received under the 1200 msthreshold.

FIGS. 5-7 summarize the results of simulations comparing the responsetime characteristics of premium users with basic users with thefollowing Premium/Basic diversity combinations: 4/2, 3/2, 2/1. Thissimulation was performed where load was provided by increasing thenumber of terminals in the channel and having 10% of the terminalsreceive the premium service. Table 1 below enumerates the configurationparameters for the simulation:

TABLE 1 Parameter Value ALOHA slots per second 1000 Round-trip Time 800milliseconds SELECT RANGE 100 MAXTRIES 10 Simulation Run Time 200 secResponse Time Threshold 1200 msec

For the simulation, a terminal generates a block of data fortransmission according to a uniform random period between 1 and 2000milliseconds after either the start of the simulation or from thecompletion of the previous transaction. As indicated by Table 1, theterminal selects a slot at random over a range of 100 slots.

The simulation results of FIGS. 5-7 are generally applicable even whenthe details of the base slotted ALOHA channel vary from those of thesimulation. The results of ALOHA simulations are not particularlysensitive to the traffic model provided the duty cycle of an individualterminal is relatively small. Also, the simulations are not sensitive towhether a single TDMA channel is used or multiple FDMA/TDMA channels.Further, the simulations are not sensitive to the retransmission backoffalgorithm or the random selection of the burst to be used when atransmission is needed.

FIGS. 5-7 clearly show that the premium terminal is offered, at moderateloading, a significant premium in terms of the percentage of burstsarriving under the response time threshold. For example, the system with4/2 diversity (as seen in FIG. 5) has a function 501 corresponding tothe premium terminal and a function 503 corresponding to the basicterminal. In FIG. 5, at a 15% load, the 99% of the premium terminal'stransactions complete under the threshold, while only 90% of the basicterminal's transactions complete under the threshold. More dramatically,at a load of 23.5%, 95% of a premium terminal's transaction completeunder the threshold compared to only 76% of the basic terminal'stransactions. Similar behavior is exhibited by the graphs of FIGS. 6 and7, in which functions 601, 701 correspond to the premium service level,and functions 603, 703 are associated with the basic service level.

Based on the results of FIGS. 5-7, it is noted that the 2/1 systemperforms best when the maximum channel load exceeds 30%; otherwise, the4/2 system may be preferable. The 3/2 may be desirable when the fractionof premium terminals exceeds 10%.

Thus, the present invention, according to an embodiment of the presentinvention, overcomes the difficulties of providing variable quality ofservice of different users within a slotted ALOHA (or diversity ALOHA)multiple access system.

FIG. 8 illustrates a computer system 800 upon which an embodimentaccording to the present invention can be implemented. The computersystem 800 includes a bus 801 or other communication mechanism forcommunicating information, and a processor 803 coupled to the bus 801for processing information. The computer system 800 also includes mainmemory 805, such as a random access memory (RAM) or other dynamicstorage device, coupled to the bus 801 for storing information andinstructions to be executed by the processor 803. Main memory 805 canalso be used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by theprocessor 803. The computer system 800 further includes a read onlymemory (ROM) 807 or other static storage device coupled to the bus 801for storing static information and instructions for the processor 803. Astorage device 809, such as a magnetic disk or optical disk, isadditionally coupled to the bus 801 for storing information andinstructions.

The computer system 800 may be coupled via the bus 801 to a display 811,such as a cathode ray tube (CRT), liquid crystal display, active matrixdisplay, or plasma display, for displaying information to a computeruser. An input device 813, such as a keyboard including alphanumeric andother keys, is coupled to the bus 801 for communicating information andcommand selections to the processor 803. Another type of user inputdevice is cursor control 815, such as a mouse, a trackball, or cursordirection keys for communicating direction information and commandselections to the processor 803 and for controlling cursor movement onthe display 811.

According to one embodiment of the invention, the process of FIGS. 3A-3Cis provided by the computer system 800 in response to the processor 803executing an arrangement of instructions contained in main memory 805.Such instructions can be read into main memory 805 from anothercomputer-readable medium, such as the storage device 809. Execution ofthe arrangement of instructions contained in main memory 805 causes theprocessor 803 to perform the process steps described herein. One or moreprocessors in a multi-processing arrangement may also be employed toexecute the instructions contained in main memory 805. In alternativeembodiments, hard-wired circuitry may be used in place of or incombination with software instructions to implement the embodiment ofthe present invention. Thus, embodiments of the present invention arenot limited to any specific combination of hardware circuitry andsoftware.

The computer system 800 also includes a communication interface 817coupled to bus 801. The communication interface 817 provides a two-waydata communication coupling to a network link 819 connected to a localnetwork 821. For example, the communication interface 817 may be adigital subscriber line (DSL) card or modem, an integrated servicesdigital network (ISDN) card, a cable modem, or a telephone modem toprovide a data communication connection to a corresponding type oftelephone line. As another example, communication interface 817 may be alocal area network (LAN) card (e.g. for Ethernet™ or an AsynchronousTransfer Model (ATM) network) to provide a data communication connectionto a compatible LAN. Wireless links can also be implemented. In any suchimplementation, communication interface 817 sends and receiveselectrical, electromagnetic, or optical signals that carry digital datastreams representing various types of information. Further, thecommunication interface 817 can include peripheral interface devices,such as a Universal Serial Bus (USB) interface, a PCMCIA (PersonalComputer Memory Card International Association) interface, etc.

The network link 819 typically provides data communication through oneor more networks to other data devices. For example, the network link819 may provide a connection through local network 821 to a hostcomputer 823, which has connectivity to a network 825 (e.g. a wide areanetwork (WAN) or the global packet data communication network nowcommonly referred to as the “Internet”) or to data equipment operated byservice provider. The local network 821 and network 825 both useelectrical, electromagnetic, or optical signals to convey informationand instructions. The signals through the various networks and thesignals on network link 819 and through communication interface 817,which communicate digital data with computer system 800, are exemplaryforms of carrier waves bearing the information and instructions.

The computer system 800 can send messages and receive data, includingprogram code, through the network(s), network link 819, andcommunication interface 817. In the Internet example, a server (notshown) might transmit requested code belonging to an application programfor implementing an embodiment of the present invention through thenetwork 825, local network 821 and communication interface 817. Theprocessor 804 may execute the transmitted code while being receivedand/or store the code in storage device 89, or other non-volatilestorage for later execution. In this manner, computer system 800 mayobtain application code in the form of a carrier wave.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to the processor 804 forexecution. Such a medium may take many forms, including but not limitedto non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 809. Volatile media include dynamic memory, such asmain memory 805. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 801.Transmission media can also take the form of acoustic, optical, orelectromagnetic waves, such as those generated during radio frequency(RF) and infrared (IR) data communications. Common forms ofcomputer-readable media include, for example, a floppy disk, a flexibledisk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM,CDRW, DVD, any other optical medium, punch cards, paper tape, opticalmark sheets, any other physical medium with patterns of holes or otheroptically recognizable indicia, a RAM, a PROM, and EPROM, a FLASH-EPROM,any other memory chip or cartridge, a carrier wave, or any other mediumfrom which a computer can read.

Various forms of computer-readable media may be involved in providinginstructions to a processor for execution. For example, the instructionsfor carrying out at least part of the present invention may initially beborne on a magnetic disk of a remote computer. In such a scenario, theremote computer loads the instructions into main memory and sends theinstructions over a telephone line using a modem. A modem of a localcomputer system receives the data on the telephone line and uses aninfrared transmitter to convert the data to an infrared signal andtransmit the infrared signal to a portable computing device, such as apersonal digital assistant (PDA) and a laptop. An infrared detector onthe portable computing device receives the information and instructionsborne by the infrared signal and places the data on a bus. The busconveys the data to main memory, from which a processor retrieves andexecutes the instructions. The instructions received by main memory mayoptionally be stored on storage device either before or after executionby processor.

Accordingly, an approach is provided performing a contention scheme thatsupports differentiated quality of service levels. Under one embodimentof the present invention, terminals communicate according to a diversityALOHA protocol, in which quality of service levels correspond to theamount of diversity. In an exemplary embodiment, a communications systemwith two-levels of service (e.g., basic and premium) can specify thatthe basic terminals operate with a diversity of two slots (i.e.,transmitting each portion of data in two different, duplicate bursts),while the premium service can specify a diversity of four slots (i.e.,transmitting each portion of data in three different, duplicate bursts).Additionally, in an exemplary embodiment, the contention channel isestablished according to a Frequency Division Multiple Access(FDMA)/Time Division Multiple Access (TDMA) scheme for providingcommunication in a satellite communications system. The abovearrangement advantageously provides a mechanism to supportQuality-of-Service (QoS) services over a contention channel.

While the present invention has been described in connection with anumber of embodiments and implementations, the present invention is notso limited but covers various obvious modifications and equivalentarrangements, which fall within the purview of the appended claims.

1. A method for providing access in a communications system, the methodcomprising: generating a plurality of bursts, wherein one of the burstsis a duplicate of another one of the bursts; transmitting the duplicatebursts over a contention channel according to a pre-determinedquality-of-service (QoS) designation; and selectively including in eachof the duplicate bursts information specifying order of the burstsrelative to another set of duplicate bursts.
 2. A method according toclaim 1, wherein the transmitting step is performed according to adiversity ALOHA protocol.
 3. A method according to claim 2, wherein theQoS designation includes a first level of service and a second level ofservice, the first level of service specifying a greater number oftransmissions of the duplicate bursts than the second level of service.4. A method according to claim 3, wherein the first level of servicespecifies one of four slots and three slots, and the second level ofservice specifies one of two slots and one slot.
 5. A method accordingto claim 1, further comprising: including in each of the duplicatebursts information identifying corresponding data within each burst. 6.A method according to claim 5, wherein the identifying information inthe including step specifies identifier information of a sourceterminal.
 7. A method according to claim 1, wherein the orderinformation in the including step specifies a modulo N sequence number.8. A method according to claim 1, wherein the communications systemincludes a satellite network supporting communication among a pluralityof terminals and a hub station, each of the plurality of terminals beingconfigured to transmit over the contention channel.
 9. A methodaccording to claim 1, wherein the contention channel is establishedaccording to a Frequency Division Multiple Access (FDMA)/Time DivisionMultiple Access (TDMA) scheme.
 10. A network device for providing accessin a communications system, the network device comprising: logicconfigured to determine a service level associated with transmission ofa burst; and a transceiver configured to transmit multiple copies of theburst over a contention channel according to the service level, whereineach of the bursts includes information specifying order of the burstsrelative to another set of bursts.
 11. A network device according toclaim 10, wherein the bursts are transmitted according to a diversityALOHA protocol.
 12. A network device according to claim 11, wherein theservice level is selected from a plurality of service levels, one of theservice levels specifying at least one of a greater number oftransmissions of the bursts than another one of the service levels. 13.A network device according to claim 12, wherein the one service levelspecifies one of four slots and three slots, and the other service levelspecifies one of two slots and one slot.
 14. A network device accordingto claim 10, wherein each of the bursts includes information identifyingcorresponding data within each of the bursts.
 15. A network deviceaccording to claim 14, wherein the identifying information specifiesidentifier information of the network device.
 16. A network deviceaccording to claim 10 wherein the order information specifies a modulo Nsequence number.
 17. A network device according to claim 10, wherein thecommunications system includes a satellite network.
 18. A network deviceaccording to claim 10, wherein the contention channel is establishedaccording to a Frequency Division Multiple Access (FDMA)/Time DivisionMultiple Access (TDMA) scheme.
 19. A system for providing access in acommunications system, the system comprising: means for generating aplurality of bursts, wherein one of the bursts is a duplicate of anotherone of the bursts; and means for transmitting the duplicate bursts overa contention channel according to a pre-determined quality-of-service(QoS) designation, wherein each of the duplicate bursts includesinformation specifying order of the duplicate bursts relative to anotherset of duplicate bursts.
 20. A system according to claim 19, wherein theduplicate bursts are transmitted according to a diversity ALOHAprotocol.
 21. A system according to claim 20, wherein the QoSdesignation includes a first level of service and a second level ofservice, the first level of service specifying a greater number oftransmissions of the duplicate bursts than the second level of service.22. A system according to claim 21, wherein the first level of servicespecifies one of four slots and three slots, and the second level ofservice specifies one of two slots and one slot.
 23. A system accordingto claim 19, wherein each of the duplicate bursts includes informationidentifying corresponding data within each of the duplicate bursts. 24.A system according to claim 23, wherein the identifying informationspecifies identifier information of the system.
 25. A system accordingto claim 19, wherein the order information specifies a modulo N sequencenumber.
 26. A system according to claim 19, wherein the communicationssystem includes a satellite network.
 27. A system according to claim 19,wherein the contention channel is established according to a FrequencyDivision Multiple Access (FDMA)/Time Division Multiple Access (TDMA)scheme.
 28. A computer-readable medium carrying one or more sequences ofone or more instructions for providing access in a communicationssystem, when executed by one or more processors, cause the one or moreprocessors to perform the steps of: generating a plurality of bursts,wherein one of the bursts is a duplicate of another one of the bursts;transmitting the duplicate bursts over a contention channel according toa pre-determined quality-of-service (QoS) designation; and selectivelyincluding in each of the duplicate bursts information specifying orderof the bursts relative to another set of duplicate bursts.
 29. Acomputer-readable medium according to claim 28, wherein the transmittingstep is performed according to a diversity ALOHA protocol.
 30. Acomputer-readable medium according to claim 29, wherein the QoSdesignation includes a first level of service and a second level ofservice, the first level of service specifying a greater number oftransmissions of the duplicate bursts than the second level of service.31. A computer-readable medium according to claim 30, wherein the firstlevel of service specifies one of four slots and three slots, and thesecond level of service specifies one of two slots and one slot.
 32. Acomputer-readable medium according to claim 28, wherein the one or moreprocessors further perform the step of: including in each of theduplicate bursts information identifying corresponding data within eachburst.
 33. A computer-readable medium according to claim 32, wherein theidentifying information in the including step specifies identifierinformation of a source terminal.
 34. A computer-readable mediumaccording to claim 28, wherein the order information in the includingstep specifies a modulo N sequence number.
 35. A computer-readablemedium according to claim 28, wherein the communications system includesa satellite network supporting communication among a plurality ofterminals and a hub station, each of the plurality of terminals beingconfigured to transmit over the contention channel.
 36. Acomputer-readable medium according to claim 28, wherein the contentionchannel is established according to a Frequency Division Multiple Access(FDMA)/Time Division Multiple Access (TDMA) scheme.
 37. A method forproviding access to a contention channel for a plurality of terminals ina communications system, the method comprising: specifying a pluralityof service levels for the plurality of terminals; and executing acommunication protocol to provide the contention channel, wherein one ofthe plurality of terminals concurrently transmits a plurality of burstsaccording to the corresponding service level, and one of the bursts is aduplicate of another one of the bursts, wherein each of the burstsincludes information specifying order of the bursts relative to anotherset of bursts.
 38. A method according to claim 37, wherein thecommunication protocol in the executing step is a diversity ALOHAprotocol.
 39. A method according to claim 38, wherein one of the servicelevels specifies at least one of a greater number of transmissions ofthe bursts than another one of the service levels.
 40. A methodaccording to claim 39, wherein the first level of service specifies oneof four slots and three slots, and the second level of service specifiesone of two slots and one slot.
 41. A method according to claim 37,wherein each of the bursts includes information identifyingcorresponding data within each of the bursts.
 42. A method according toclaim 41, wherein the identifying information specifies identifierinformation of the one terminal.
 43. A method according to claim 37,wherein the order information specifies a modulo N sequence number. 44.A method according to claim 37, wherein the communications systemincludes a satellite network.
 45. A method according to claim 37,wherein the contention channel is established according to a FrequencyDivision Multiple Access (FDMA)/Time Division Multiple Access (TDMA)scheme.
 46. A computer-readable medium carrying one or more sequences ofone or more instructions for providing access to a contention channelfor a plurality of terminals in a communications system, when executedby one or more processors, cause the one or more processors to performthe steps of: specifying a plurality of service levels for the pluralityof terminals; and executing a communication protocol to provide thecontention channel, wherein one of the plurality of terminalsconcurrently transmits a plurality of bursts according to thecorresponding service level, and one of the bursts is a duplicate ofanother one of the bursts, wherein each of the bursts includesinformation specifying order of the bursts relative to another set ofbursts.
 47. A computer-readable medium according to claim 46, whereinthe communication protocol in the executing step is a diversity ALOHAprotocol.
 48. A computer-readable medium according to claim 47, whereinone of the service levels specifies a greater number of transmissions ofthe bursts than another one of the service levels.
 49. Acomputer-readable medium according to claim 48, wherein the first levelof service specifies one of four slots and three slots, and the secondlevel of service specifies one of two slots and one slot.
 50. Acomputer-readable medium according to claim 46, wherein each of thebursts includes information identifying corresponding data within eachof the bursts.
 51. A computer-readable medium according to claim 50,wherein the identifying information specifies identifier information ofthe one terminal.
 52. A computer-readable medium according to claim 46,wherein the order information specifies a modulo N sequence number. 53.A computer-readable medium according to claim 46, wherein thecommunications system includes a satellite network.
 54. Acomputer-readable medium according to claim 46, wherein the contentionchannel is established according to a Frequency Division Multiple Access(FDMA)/Time Division Multiple Access (TDMA) scheme.
 55. A method forcommunicating over a shared medium, the method comprising: receiving oneor more bursts over a contention channel of the shared medium from aterminal, wherein the bursts are duplicates, the quantity of burstsbeing dependent on a pre-determined quality-of-service (QoS) designationassociated with the terminal; and selectively discarding all but one ofthe bursts, wherein each of the duplicate bursts in the receiving stepincludes information specifying order of the bursts relative to anotherset of duplicate bursts.
 56. A method according to claim 55, wherein thecontention channel is established according to a diversity ALOHAprotocol.
 57. A method according to claim 56, wherein the QoSdesignation includes a first level of service and a second level ofservice, the first level of service specifying a greater number oftransmissions of the duplicate bursts than the second level of service.58. A method according to claim 57, wherein the first level of servicespecifies one of four slots and three slots, and the second level ofservice specifies one of two slots and one slot.
 59. A method accordingto claim 55, wherein each of the duplicate bursts in the receiving stepincludes information identifying corresponding data within each burst.60. A method according to claim 59, wherein the identifying informationspecifies identifier information of a source terminal.
 61. A methodaccording to claim 55, wherein the order information specifies a moduloN sequence number.
 62. A method according to claim 55, wherein thecontention channel is established according to a Frequency DivisionMultiple Access (FDMA)/Time Division Multiple Access (TDMA) scheme. 63.A computer-readable medium carrying one or more sequences of one or moreinstructions for communicating over a shared medium, when executed byone or more processors, cause the one or more processors to perform thesteps of: receiving one or more bursts over a contention channel of theshared medium from a terminal, wherein the bursts are duplicates, thequantity of bursts being dependent on a pre-determinedquality-of-service (QoS) designation associated with the terminal; andselectively discarding all but one of the bursts, wherein each of theduplicate bursts in the receiving step includes information specifyingorder of the bursts relative to another set of duplicate bursts.
 64. Acomputer-readable medium according to claim 63, wherein the contentionchannel is established according to a diversity ALOHA protocol.
 65. Acomputer-readable medium according to claim 64, wherein the QoSdesignation includes a first level of service and a second level ofservice, the first level of service specifying a greater number oftransmissions of the duplicate bursts than the second level of service.66. A computer-readable medium according to claim 65, wherein the firstlevel of service specifies one of four slots and three slots, and thesecond level of service specifies one of two slots and one slot.
 67. Acomputer-readable medium according to claim 63, wherein each of theduplicate bursts in the receiving step includes information identifyingcorresponding data within each burst.
 68. A computer-readable mediumaccording to claim 67, wherein the identifying information specifiesidentifier information of a source terminal.
 69. A computer-readablemedium according to claim 63, wherein the order information specifies amodulo N sequence number.
 70. A computer-readable medium according toclaim 63, wherein the contention channel is established according to aFrequency Division Multiple Access (FDMA)/Time Division Multiple Access(TDMA) scheme.